Process for producing nickel-magnesium product by powder metallurgy



3,385,696 PRUCESS FUR PRODUCING NIQKEL-MAGNESIUM I'RODUQT BY PGWDERMETALLUR-GY John Oliver Hitchcock, Underriver, near Sevenoalss, HaroldWilliam George Hignett, Cobham, and Norman John Williams, Birmingham,England, assignors to The international Nickel (Iompany, Inc., New York,N.Y., a corporation of Delaware N0 Drawing. Filed May 10, 1965, Ser. No.454,652 Claims priority, application Great Britain, May 13, 1964,20,019/ 64 5 Claims. (Cl. 75-430) The present invention is directed tothe production of nickel-magnesium agents particularly adapted for thepurpose of introducing magnesium into molten cast iron and, moreparticularly, to a powder metallurgy method for the production of suchagents.

In recent years the introduction of magnesium into molten iron has cometo be widely practised, both for the production of spheroidal graphitecast iron, in which the graphite is rendered spheroidal by means ofretained magnesium, and for other purposes. It is now well known that toensure that the graphite formed in cast iron, either on solidificationor during a graphitizing heat treatment, is spheroidal, generallyrequires a retained magnesium content of up to about 0.1% e.g. 0.04 to0.08%.

The utility of an addition material for introducing magnesium intomolten iron may be measured in terms of its efficiency, defined asfollows:

Efficiency Mg recovery X Mg content of alloy /2) 100 Magnesium cannot beintroduced as such into molten iron because of the violence of thereaction that takes place, and the regular practice is to introduce itas an alloy with another metal. All the alloys comomnly used for thepurpose are made by melting, and the production of the alloys in thisway is very troublesome. Invariably some of the magnesium and othermetals is lost in the recess; it is impossible to exclude dross andinclusions from the alloys; the magnesium in the alloys tends tosegregate so that the composition of the alloys is not constant; and thealloys when formed have to be broken into small pieces before they canbe added to molten iron. In the course of the breaking, substantialamounts of the alloys are lost as a result of the production of unusablefine particles. Despite all these disadvantages, alloys made by meltingare those regularly used.

Of these alloys formed by melting, those of nickel and magnesium affordcertain advantages, and in particular nickel is extremely effective inmoderating the violence of the reaction between magnesium and molteniron. Moreover it is often a very useful constituent of the cast ironformed. The standard nickel-containing alloy in use contains from 14 to16% magnesium. This alloy has many advantages. In particular, itsreaction with molten iron, though spectacular, is not violent; and itsdensity is greater than that of the molten iron so that it sinks intothe melt and remains submerged during reaction.

Nickel-magnesium alloys containing a higher proportion of magnesium havelower densities than the 15% magnesium alloy, and to avoid excessivelosses of magnesium they need to be held below the surface of the meltby a plunger while they react with the molten iron. When they are addedby plunging, the efficiency of such nickelnited States Paten magnesiumalloys is high, and generally higher than that obtained by a directaddition of the standard 15 magnesium alloy. Nevertheless thesehigh-magnesium alloys are not used to any great extent, largely becausethey are very difficult to produce commercially by melting techniques.Melting losses are high, and segregation of magnesium in the castnickel-magnesium ingots readily occurs.

Various proposals have also been made to produce magnesium-containingaddition agents by compacting magnesium powder with other powders, withor without subsequent sintering of the compacts. When the compacts areadded as such to the molten iron, the magnesium is inadequatelyprotected from the iron, and the magnesium recovery and efficiency bothtend to be low. When the compacts are sintercd, there is loss ofmagnesium during the sintering, with resultant difiiculties incontrolling the composition of the addition agent.

We have now discovered a method for producing nickelmagnesium additionagents by powder metallurgy which overcomes the aforementioned problemsand provides agents which are metallurgically clean, which can beproduced in bodies of regular, controllable size, weight and magnesiumcontent and which are effective addition agents for the purpose ofintroducing magnesium into molten cast iron even when the magnesiumcontent thereof is substantially higher than that which has beenheretofore generally accepted as being tolerable on a practical level.

It is an object of the present invention to provide a powder metallurgymethod for the production of nickelmagnesium bodies particularly usefulfor the introduction of magnesium into molten cast iron.

It is another object of the present invention to provide a method forproducing nickel-magnesium bodies by powder metallurgy which aremetallurgically clean and which have a controlled substantially uniformmagnesium content.

Still another object of the present invention is to provide by powdermetallurgy nickel-magnesium bodies having improved additioncharacteristics when employed to introduce magnesium into molten castiron.

It is a further object of the present invention to produce by powdermetallurgy improved nickel-magnesium bodies of controlled size andweight which enable calculation of desired magnesium additions simply bycounting the quantity of nickel-magnesium bodies to be employed.

Other objects and advantages of the invention will become apparent fromthe following description.

We have found that useful nickel-magnesium addition agents can beprepared without melting from particles of magnesium coated with nickel.Our invention comprises compacting together particles of nickel-coatedmagnesium powder with or without other particles in a proportion suchthat the magnesium constitutes from 10 to 50% of the compacts by weight.Higher proportions of magnesium lead to undesirably violent reactionwith molten iron and low recoveries of magnesium, while if the magnesiumcontent is less than 10% such a large proportion of the addition agentmust be added to the iron to introduce a given amount of magnesium asundesirably to lower the temperature of the molten iron, and alsounnecessarily to increase the cost.

The compacts may either be used as such or may be sintered beforeaddition to molten iron. In the unsintered compacts the nickel coatingon the particles serves to restrain the reaction of the magnesium withthe molten iron, while in the sintered compacts it reacts with themagnesium during sintering to form intermetallic compounds and solidsolutions that react with only moderate vigor with the molten iron. Theintimate association of the magnesium and the nickel of the coatingprovide very favorable conditions for their interdiifusion and alloyingduring sintering without loss of magnesium. The thickness of the coatingis thus of importance both in green (unsintered) compacts and when theyare sintered. In unsintered compacts a thicker coating delays contactbetween the magnesium and the molten iron to a greater extent than athin coating, thereby damping down the subsequent reaction and giving ahigher magnesium recovery. In making sintered compacts a certain amountof disruption of the coating to allow escape of molten metal is neededin order to sinter the particles together. Hence the nickel coating mustneither be so thick that no disruption occurs nor be so thin thatcomplete disruption occurs, thereby occasioning partial melting of thecompacts and loss of material. The thickness of the coating depends bothon the proportion of nickel to magnesium and on the size of theparticles. For a given proportion of nickel and magnesium the thicknessof the nickel coating decreases as the size of the base magnesiumparticle decreases. In practice magnesium particles smaller than 200mesh BSS (0.075 mm.) cannot be used for coating, and for sizes down tothis adequate coating thicknesses can be obtained when the coatedparticles contain from 50 to 90% of nickel.

We prefer to form the compacts wholly from nickelcoated magnesiumpowder, since this gives the highest degree of uniformity ofcomposition. If desired, however, the nickel-coated magnesium powder maybe admixed with powder of other constituents such as nickel, iron,silicon and copper commonly used as diluents in magnesium additionagents in order to moderate their reaction with molten iron. The use ofany such diluent powder gives rise to risk of segregation in thecompacts to the extent that the size and shape of the particles differfrom those of the nickel-coated magnesium particles, but on the otherhand a diluent powder may assist in moderating the reaction and therebypermit the nickel coating to be thinner than is otherwise required. Infact, with a large proportion of the diluent the weight of the nickel inthe nickel-coated magnesium powder may be reduced, provided always thatthe magnesium particles are still completely coated, and indeed theproportion of nickel in the coated particles may then be as low as Thus,the compacts may contain substantial additional amounts of nickelpowder, to bring the total nickel content of the compacts up to as muchas 90% of the weight of the compacts, the maximum amount of added nickelbeing set by the need for an adequate coating of nickel on the magnesiumparticles while maintaining a sufiicient magnesium content in thecompact. The resulting compacts have the advantage of having no otherconstituents than nickel and magnesium. Iron, which is not as effectiveas nickel in moderating the violence of the reaction, may be present inamounts up to 30% of the weight of the compacts. Silicon, which has amoderating power between that of nickel and iron, and which is alsouseful as a graphitizing agent, may be present up to 70% of the weightof the compacts. Iron and silicon in the compacts may, if desired, bealloyed together as ferro-silicon powder.

Copper, the other common diluent metal, is an excellent moderator of theviolence of the reaction, but in large amounts adversely affects theformation of spheroidal graphite in cast iron, and for this reason theproportion of any copper in the compacts should not exceed 30%.

The nickel coatings on the particles may be formed by any convenientmethod, but we prefer to deposit the nickel by the thermal decompositionof nickel carbonyl. In this way a continuous layer of nickel is formedover the particle surface and may be built up to any desired thickness,thus enabling the proportion of nickel to magnesium in the coatedparticles to be varied over a very wide range.

The green strength of the compacts is also afiected by the particlesize, decreasing as the particles become coarser, and for this reason inunsintered compacts the coated powder is preferably not larger than 72mesh BS8 (0.2 mm), and most advantageously not larger than mesh (0.18mm). The strength of unsintered compacts may be increased, though at theexpense of some loss in efiiciency, by incorporating a small proportionof binder, e.g. a synthetic resin, either throughout the mix or as asurface layer. Alternatively, the compacts may be enclosed in bags orcoverings, e.g. of polythene, to facilitate their handling. The highestefliciency is obtained when the coated powder does not contain anysubstantial proportion of particles smaller than mesh (0.1 mm.).

When the compacts are sintered, coarser particles, e.g. from 10 to 60mesh 1388 (1.7 to 0.25 mm.), can be employed. However, sinteringintroduces a further processing operation with attendant increase incost, and we prefer to employ the unsintered compacts.

The magnesium may be alloyed with small amounts of elements, e.g. up to1% of silicon, added before it is coated to render it brittle and aid inthe production of magnesium powder by pulverization of a cast ingot ofmagnesium. Care must however be taken to avoid any additions that have adeleterious effect on the formation of spheroidal graphite in cast iron.

The addition agents of the invention have many advantages compared withmaterials of similar composition prepared by melting, casting andcrushing to size. Thus they can readily be made by standardpowdermetallurgical procedures without appreciable loss of nickel ormagnesium, whereas in preparing nickel-magnesium alloys by meltingsubstantial losses are encountered. This is particularly advantageous inthe case of addition agents of high magnesium content, e.g. about 30% or40% to about 50%. For example, in melting a nickel-magnesium alloycontaining 15% by weight of magnesium about 7% of the materials chargedis lost as dross or otherwise during the melting process, and the lossesin melting such alloys of higher magnesium content are still greater. Inaddition, when large pieces such as cakes, of such melted and castmaterial are crushed and sized to yield a marketable product furtherlosses of material in the form of unusable fines are encountered.Furthermore, the melted alloys inevitably are somewhat dirty, i.e.,contain dross and inclusions, whereas the compacts are clean. Unlike thebroken pieces of the melted alloys, the compacts are readily made ofregular and uniform shape and size, are easily packed, and have areliably reproducible composition. Thus, they can be made to contain anaccurately predetermined amount of magnesium, so that a given magnesiumaddition may readily be calculated by simply counting out the number ofcompacts to be added to a given metal.

When the addition agents of the invention are added to molten iron, e.g.for the production of spheroidal graphite cast iron, in a mannerappropriate to their density, i.e. by plunging in the case of compactshaving a magnesium content exceeding 15%, magnesium recoveries andefficiencies similar to or even better than those for melted alloys ofthe same composition are obtainable.

In order to obtain these advantages it is essential that the particlesof magnesium powder used to form the compacts are coated with nickel.Unsintered addition bodies made by compacting mixtures of nickel anduncoated magnesium powders give lower magnesium recoveries when added tomolten iron than unsintered compacts of similar composition made fromnickel-coated magnesium powder, and attempts to sinter mixtures ofnickel and magnesium powders lead to losses of magnesium owing to thelow-melting point of magnesium and to the formation of a lowmelting-point eutectic.

In order to give those skilled in the art a better appreciation of theadvantages of the invention, the following examples will now be given:

Example I Magnesium powders of commercial purity having particle sizesof about 60 mesh BSS, in the range minus 85 to plus 120 mesh and in therange minus 100 to plus 150 mesh respectively, were coated with nickelby the thermal decomposition of nickel carbonyl to provide coatedpowders containing, by weight, about 60% nickel, the balance beingmagnesium. The initial powders were elongated in particle shape, someangular particles being present in the 60 mesh powder. The powders werebriquetted at a pressure of about 30 long tons per square inch toproduce strong cylindrical pellets having a diameter of about one inchand a height of about 0.75 inch. About 0.4% by weight of each of theresulting briquettes was introduced into separate portions of the samemolten iron containing 3.81% carbon, 1.65% silicon, 2% manganese, 0.19%phosphorous and 0.01% sulfur while the molten iron was at a temperatureof 1500 C. in each instance. Metal from each of the treated melts wascast and analyzed for magnesium. The results of these tests aresummarized in the following Table I, which shows the progressive fall inrecovery and efficiency with decreasing particle size. Each of thecastings had a satisfactory spheroidal graphite structure.

Magnesium powder containing about 0.5% silicon was coated with nickel bythe thermal decomposition of nickel carbonyl to provide coated particlescontaining about 60% nickel, the balance being magnesium. The magnesiumpowder had a particle size of about 36 mesh and had a regular, angularshape. Portions of the coated powder were coni'pacted at a pressure ofabout 30 long tons per square inch to provide cylindrical pellets havinga diameter of about one inch and a height of about 0.75 inch. About 0.4%by weight of the as-pressed compacts were plunged into two otherportions of the molten iron at 1500 C. as in Example I. Residualmagnesium contents of 0.043% and 0.051%, representing an averagemagnesium recovery of about 33.7% and an average efficiency of about13.5%, respectively, were obtained. The compacts obtained in thisinstance were noticeably more friable than those obtained using finerpowders as set forth in Example I and the lower magnesium recoveryobtained was attributed to this factor.

Example III Atomised magnesium powder having a particle size range of 22to 44 mesh BSS was coated with nickel by thermal decomposition of nickelcarbonyl. The magnesium powder particles were rounded in shape, with afew re-entrants and the nickel coatings were from 20 to 30 microns thickand covered the whole surface fairly uniformly and filled theirregularities. The analysed nickel content of the powder was 58.8%.Portions of the coated powder were compacted under a pressure of 30 tonsper square inch to form cylindrical pellets about one inch in diameterand about 0.75 inch high. These had fair green strength, but tended tocrumble at the edges. Their strength was not significantly improved byheating at temperatures below 400 C., above which interdiffusion beganto occur between the nickel and the magnesium as a preliminary tosintering.

On plunging 0.5% by weight of the as-pressed pellets into molten ironcontaining 3.68% carbon, 1.4% silicon,

d 0.2% manganese, 0.04% sulfur and 0.023% phosphorus, the balance beingiron, at a temperature of 1500 C. a moderate reaction occurred and theiron when cast contained 0.066% magnesium and had a satisfactoryspheroidal graphite structure.

Example IV Pellets prepared as described in Example III were heated for30 minutes at 650 C. in an atmosphere of dry hydrogen. Under theseconditions the magnesium center of the particles just melted, and thenickel and magnesium reacted to form the intermetallic compound Mg Niand the eutectic Mg/Mg Ni. Substantial amounts of the nickel coatingremained unreacted and leakage of the molten metal through the nickelcoating sintered the particles together and greatly increased thestrength of the bodies.

On plunging 0.5% by weight of the sintered pellets into molten iron at1500 C. the reaction was brisk but not so vigorous as to be hazardous.The iron when cast contained 0.042% magnesium and had satisfactoryspheroidal graphite structure.

Example V Further compacted pellets prepared as described in Example IIIwere heated for one hour in dry hydrogen at 725 C. i.e., below thetemperature (760 C.) of peritectic formation of the intermetalliccompound Mg Ni. Substantially all the nickel reacted with the magnesiumto form Mg Ni and Mg/Mg Ni eutectic and the identity of the individualparticles was almost lost in the sintered mass. Sintered pellets formedin this way, added in an amount of 0.5 to a further portion of themolten iron described in Example III at 1500" C. by plunging, reacted ina similar manner to the unsintered pellets to give an iron which, whencast, contained 0.059% magnesium and has a satisfactory spheroidalgraphite structure. An attempt to sinter the pellets of Example III at ahigher temperature, by heating for 15 minutes at 850 C., led to partialmelting of the pellets and loss of material. The resulting bodies werenot tested by addition to molten iron. For purposes of comparison, 0.5%of an experimental alloy made by a melting process and containing 60%nickel and 40% magnesium was also used to treat another portion of thesame molten iron by plunging. The reaction on plunging was notover-vigorous, and the resultant cast iron contained about 0.05%magnesium and had a satisfactory spheroidal graphite structure.

Example VI A mixture of 36.4 parts by weight of the nickel-coatedmagnesium powder used in Example III with 63.6 parts by weight ofcarbonyl nickel powder was compacted at 30 tons per square inch as inExample 111 to pellets having the composition nickel-15% magnesium andthe pellets were sintered by heating at 850 C. for 15 minutes. Theresulting sintered bodies had a duplex structure consisting of nickeland the intermetallic compound MgNi Despite the high temperatureemployed for sintering, the presence of the nickel powder substantiallyprevented loss of magnesium. When 1.0% of these bodies were added tomolten iron at 1450 C. by tapping the iron on to the addition bodies,the reaction was not violent. Castings poured from the treated melt hada magnesium content of 0.073% and had a satisfactory spheroidal graphitestructure. For purposes of comparison, a standard alloy of 85% nickell5%magnesium made by adding magnesium to molten nickel was similarly addedto another portion of the same iron melt. The vigor of the reaction wassimilar to that occurring with the sintered material, and the resultingiron contained about 0.052% magnesium, and had a satisfactory spheroidalgraphite structure. The results of Examples III to VI are summarised inthe following Table II:

nesium, with the balance essentially nickel, said compact being madefrom an initial powder mix comprising mag- TABLE II Residual MagnesiumElli- Ex. No. Addition Material Addition Method Magnesium Recovery,ciency, Content, percent percent percent In 40% Mg, 60% Ni compactedPlungcd 0. 066 48 19 1V 40% Mg, 60% Ni sintercd (550 C ..d 0. 042 3G 14V 140% Mg, 60% Ni sintcred 725 (3..... do 0. 059 44 18 "140% Mg, (50%Nin1clted .d 0.05 4t) 16 V1 [15% Mg, 85% Ni sintered 850 C Iron tappedon to addition material 0. 073 0!) (%Mg,85%Nimclted.. do 0.052 55 8 Itwill be understood that the magnesium-containing addition agentaccording to the invention is also useful for treating molten metalswith magnesium for purposes other than the production of ductile iron.Such purposes include the desulphurisation of cast iron and of steel.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariaiions may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

11. In the process for producing ductile iron wherein magnesium isemployed to effect the occurrence of graphite in the spheroidal form,the improvement Which comprises introducing magnesium into molten castiron as a briquetted agent containing about 10% to about 50% magnesium,with the balance essentially nickel made from an initial powder mixcomprising magnesium particles coated with nickel.

2. In the process for producing ductile iron wherein magnesium isemployed to effect the occurrence of graphite in the spheroidal form,the improvement which comprises introducing magnesium into molten castiron as a powder metallurgical compact containing about 10% to about 50%magnesium, with the balance essentially nickel, said compact being madefrom an initial powder mix comprising magnesium particles coated withnickel and having a particle size of not more than about 72 mesh BSS.

3. In the process for producing ductile iron wherein magnesium isemployed to effect the occurrence of graphite in the spheroidal form,the improvement which comprises introducing magnesium into molten castiron as a ower metallurgical compact containing about 40% magnesiumparticles coated with nickel and having a particle size of not more thanabout 85 mesh B85.

4. In the process for producing ductile iron wherein magnesium isemployed to effect the occurrence of graphite in the spheroidal form,the improvement which comprises introducing magnesium into molten castiron as a sintered powder metallurgical compact containing about 10% toabout magnesium, with the balance essentially nickel, said compact beingmade from an initial powder mix comprising magnesium particles coatedwith nickel and having a particle size from about 10 mesh to about meshB85.

5. An addition agent for adding magnesium to molten metals consisting ofcompacts in which the magnesium is present as nickel-coated magnesiumpowder having a particle size not greater than 60 mesh BSS, and saidmagnesium amounts to from 10 to 50% by weight of the compacts, with thebalance essentially nickel.

References Cited UNITED STATES PATENTS 1,555,978 10/1925 Hunt -532,839,393 6/1958 Ka'wabata -3 75130 2,873,188 2/1959 Bieniosek 751302,881,068 4/1959 Bergh 7553 2,930,712 3/1960 Homer et al 75-.5 2,935,3945/1960 H'iler 75.5 2,988,444 6/1961 Hurum 7553 2,988,445 6/1961 Hurum7558 3,151,975 10/1964 Madaras 75129 X 3,298,801 l/1967 Goodrich et al75130 X 3,314,787 4/1967 Goodrich et a1. 75-130 X 3,336,118 8/1967Newitt 7553 X HYLAND BIZOT, Primary Examiner.

H. W. TARRING, A ssistanl Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,385,696 May 28 1968 John Oliver Hitchcock et al.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

In the heading to the printed specification, line 10, "May 13 196 4,"should read May 6 1965 Column 1, line 43, "comomnly" should readcommonly Column 7,

line 50, "power" should read powder Signed and sealed this 11th day ofNovember 1969.

(SEAL) Attest:

Edward Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

1. IN THE PROCESS FOR PRODUCING DUCTILE IRON WHEREIN MAGNESIUM ISEMPLOYED TO EFFECT THE OCCURRENCE OF GRAPHITE IN THE SPHEROIDAL FORM,THE IMPROVEMENT WHICH COMPRISES INTRODUCING MAGNESIUM INTO MOLTEN CASTIRON AS A BRIQUETTED AGENT CONTAINING ABOUT 10% TO ABOUT 50% MAGNESIUM,WITH THE BALANCE ESSENTIALLY NICKEL MADE FROM AN INITIAL POWDER MIXCOMPRISING MAGNESIUM PARTICLES COATED WITH NICKEL.
 5. AN ADDITION AGENTFOR ADDING MAGNESIUM TO MOLTEN METALS CONSISTING OF COMPACTS IN WHICHTHE MAGNESIUM IS PRESENT AS NICKEL-COATED MAGNESIUM POWDER HAVING APARTICLE SIZE NOT GREATER THAN 60 MESH BSS, AND SAID MAGNESIUM AMOUNTSTO FROM 10 TO 50% BY WEIGHT OF THE COMPACTS, WITH THE BALANCEESSENTIALLY NICKEL.